Is there any difference between ConvNet and CNN?
Both terms just mean convolutional neural network. I don't believe there is any particular reason to choose one over the other: ConvNet is slightly easier to say out loud and CNN is slightly shorter to write, but there is absolutely no difference in meaning.
For some contrasting examples in the literature, the EfficientNet paper chooses the term ConvNet and this paper on AlexNet chooses CNN. They are talking about the same sorts of neural networks though!